Mobile devices, such as personal computers, digital cameras, and mobile phones, have been recently used widely, and have been demanded to have a small size, a low profile, a light weight, high resolution, and multiple functions. Semiconductor elements used for such devices accordingly have had a small package and are mounted in a three-dimensional structure. In order to provide the semiconductor devices with a small size in a three-dimensional structure, a board having a cavity, i.e., a recess therein is used.
FIGS. 15A and 15B are sectional views conventional circuit board 127 with a cavity described in Patent Document 1 for illustrating a method of manufacturing the circuit board.
As shown in FIG. 15A, upper substrate 123, connection layer 121, and lower substrate 122 are stacked such that connection layer 121 is located between lower substrate 122 and upper substrate 123 while electrodes and openings are aligned, thereby providing circuit board 127. After that, sheet 126 is placed on upper substrate 123. Sheet 126 includes removable layer 124 and thermoplastic resin layer 125 provided on removable layer 124. Thermoplastic resin layer 125 is fluidized with heat.
As shown in FIG. 15B, sheet 126 is heated and pressure-bonded onto upper substrate 123. Heat and pressure applied to sheet 127 fluidize thermoplastic resin layer 125 of sheet 126 to fill recess 128 and prevent connection layer 121 from moving into recess 128. Therefore, upper substrate 123, connection layer 121, and lower substrate 122 are stacked before connection layer 121 moves. After that, sheet 126 is peeled off, thus providing circuit board 127.
In circuit board 127, edge 129 of upper substrate 123 constituting an edge of recess 128 has a nearly right angle, which prevents components to from being mounted around the edge of recess 128 easily. Besides, removable layer 124 can be easily broken due to the right-angled edge 129. Upon sheet 126 being removed, removable layer 124 cannot be completely removed from recess 128 and edge 129.
As shown in FIG. 15B, sheet 126 pressure-bonded onto circuit board 127 deforms to have a shape identical to the shape of recess 128. Sheet 126 is removed from circuit board 127 with its shape maintained, hence being prevented from being removed when recess 128 is deep.
FIG. 16 is a sectional view of another conventional circuit board 501 manufactured in a manner similar to that of circuit board 127 shown in FIGS. 15A and 15B. FIG. 17 is a sectional view of circuit board 501 for illustrating a method of manufacturing circuit board 501. In FIGS. 16 and 17, components identical to those of circuit board 127 shown in FIGS. 15A and 15B are denoted by the same reference numerals, and their description will be omitted. In circuit board 501 shown in FIG. 16, edge 129 of upper substrate 123 constituting an edge of recess 128 has a nearly right angle. If mask 130 is placed in recess 128 for printing solder on land 128A on a bottom of recess 128, mask 130 easily contact edge 129 of upper substrate 123 as shown in FIG. 17. This contact often causes damage to mask 130 when mask 130 is placed in recess 128 or is removed from recess 128. In particular, if mask 310 is not aligned to land 128A accurately, mask 130 often contacts the edge of recess 128 and provides the edge with damage. Further, this nay prevents mask 130 from being placed easily in recess 128.
Patent Document 1: JP 63-90158A